Vacuum formed films typically have a plurality of apertures that allow liquids and gases to pass through the film. Such films may be incorporated into disposable products, such as feminine hygiene products, diapers, incontinent products, hospital pads, bedding, breathable clothing and/or sleeping bag linings, for example. Such products are typically composite structures comprising a top sheet for use adjacent to the skin and an absorbent layer. An example of an apertured film for making disposable diapers is disclosed in U.S. Pat. No. 3,814,101, which is hereby incorporated by reference.
One method of forming apertures in thin webs or plastic film is by vacuum film forming, see U.S. Pat. No. 3,054,148 which is hereby incorporated by reference, for example. In this process, the plastic film lays on a rotating screen having a plurality of holes. The film passes over a vacuum chamber as the screen rotates creating a pressure differential on either side of the plastic film. The pressure differential causes the film to rupture at the holes in the screen to form the apertures. The holes in the screen may be in a specific pattern or shape that transfers onto the film in the process. In this process, the heated film is supported by a screen and a vacuum applied to the underside of the perforated screen. Holes are pulled in the film in the direction of the vacuum beneath the screen thereby forming tapered capillaries in the film.
Many methods and apparatuses for preparing plastic films comprising apertures have been developed, examples include U.S. Pat. Nos. 4,155,693; 4,252,516; 3,709,647; 4,151,240; 4,319,868; 4,388,056; 4,950,511; 4,948,638; 5,614,283; and 5,887,543, which are all hereby incorporated by reference.
Conventional vacuum film forming process may comprise extruding molten polymeric materials such as polyethylene or other polyolefin through a die to form a hot melt web or by laying a previously formed thin web or plastic film on a screen. The hot melt web of polymer lays on a rotating screen mounted on a stationary vacuum drum. The vacuum drum has an axial slot and a set of seals extending longitudinally along the length of its inside surface, beneath the area where the web of plastic impinges on the screen or molding element. A vacuum is pulled on the inside the screen to form the apertures in the film through the holes in the screen. After the aperture is formed, air flowing through the film cools the film and sets the shape of the aperture.
The physical shape of the screen determines the geometric pattern of apertures on the film and thus contributes to its aesthetic, tactile and mechanical properties. U.S. Pat. Nos. 5,897,543; 5,718,928; 5,591,510; and 5,562,932, which are all hereby incorporated by reference, describe films, screens, and methods of forming films having apertures that form capillaries extending through the film and the capillaries are disposed at an angle from about 5° to about 60° with respect to a plane that is perpendicular to a surface of the film. Accordingly, the films will have a masking characteristic such that the absorbent material is not as visible to a user as when the capillaries are perpendicular to the surface of the film.
There is a need for a thin web comprising apertures acting as preferential flow valves. There is a need for a thin web that provides a significantly greater resistance to flow of a liquid through the web in one direction as compared the flow of the same liquid in the opposite direction.